Plasticizing of glassine papers



Patented July 9, 1946 Puis'ncrzmo or cmssma rsraas Herbert B. Wenberg, De Pere, Wis.

N Drawing. Application June 4, 1943,

' Serial No. 489,716

14 Claims. (Cl. 8-1162) My invention relates to the softening or plasticizing of papers, particularly papers made from highly beaten or hydrated" fibers such as glassine.

Paper sheets produced from highly hydrated problem of brittleness of glassine and similar papers. The conventional practice has been to employ agents such as glycerine, ethylene glycol, diethylene glycol, invert sugar, sorbitol, corn sugar, corn syrup and hygroscopic salts such as calcium chloride. It has been thought that these agents, because of their hygroscopic properties, would hold moisture in the glassine sheets and thereby prevent brittleness from developing at low humidities. In actual fact, with the possible exception of glycerine, these substances serve merely to retard the drying out of the glassine sheets at low humidities and when such sheets do dry out, especially during the winter months, they become exceedingly brittle. Glycerine does have a tendency to soften glassine sheets at low humidities but the effect is only slight. Apart from its lack of adequate softening properties, glycerine has other disadvantages among which are its impairment of the transparency of the glassine sheets and the fact that it volatilizes from the glassine sheet although, to be sure, at a relatively slowrate. Glycerine has the still further disadvantage, and this it shares in common with at least most of the softening agents mentioned above, in that when the glassine sheets are coated,-after manufacture, with thermoplastic coatings, as is common practice with much of the glassine produced, the glycerine does not fully satisfactorily retain or hold the moisture in the glassine sheets as they pass through the thermoplastic coating compositions.

It has also heretofore been uggested to soften paper by'means of aqueous solutions containing low concentrations of urea and higher molecular weight alkyl sulphates such as sodium cetyl sul phate. Again-for the softening of paper such as ing of glassine.

glassine, the suggestion has been made to employ aqueous solutions of unreacted urea in ad-' mixture with glycerine or other hygroscopic substances such as calcium chloride, magnesium chloride, sodium acetate, sodium sulphate, and the like. Efforts have also been made to utilize aqueous solutions of urea alone for the soften- Based upon the work which I have done, none of these procedures is adequately satisfactory.

Still other plasticizers which have been employed are various oils, sulphonated oils, and certain waxes but all of such materials, although of some utility, leave much to be desired.

I have discovered that glassine paper may be softened or plasticized with exceptionally satisfactory results by the utilization of certain addition, compounds of urea, all as is hereinafter set forth in detail.

, The addition compounds which are utilized in accordance with the present invention are derived from urea and Water-soluble nitrates in certain specified ratios. Among the nitrates which are utilized to form addition compounds with the urea are, for example, calcium nitrate and the alkali metal nitrates, such as sodium nitrate, potassium nitrate and lithium nitrate. I prefer to use the alkali metal nitrates, sodium nitrate having been found tobe unusually satisfactory. I

The molal ratios of urea to the nitrates, employed in the formation of .the addition compounds which are utilized in accordance with my present invention, comprise not substantially less than three mols of urea for each moi of nitrate. In the case of the alkali metal nitrates, three mols of urea should'be used to eachmol of the said nitrates to produce the addition compounds.

I have found it to be important to utilize the stated molal ratios of the urea and nitrates, in

the formation of the addition compounds which are utilized in accordance with my present invention, since, if applied to the paper in ratios materially varying from those set forth, where the addition compounds are utilized in aqueous solutions as hereinafter described, upon drying the paper, the crystals'of whichever chemical is in excess crystallize out on the surface of the sheet. Where the addition compounds are applied in molten form to the paper, as hereinafter described, if the molal ratios are not of the order set forth hereinabove, the chemical which is in excess of the said molal ratios crystallizes out first and serves as a harmful or objectionable filler in the sheet.

ing point of this particular addition compond is 83 degrees C. It may be pointed out that changing the molal ratio of the urea to the soduim nitrate does not change the meltin int or analysis of the liquid which solidifies at 83 degrees.

C., but whichever chemical is in excess of the stated molal ratio of three mols of urea to one mol of sodium nitrate will crystallize out before the temperature drops to 83 degrees C. The solubility of this addition compound of urea and sodium nitrate is approximately 240 grams per 100 cc. of water at room temperature, which is materially higher than the sum of the solubilities based upon urea and sodium nitrate considered individually.

In softening glassine paper in accordance with the present invention, the addition compounds,

exemplified by the urea-sodium nitrate product.

produced as described hereinabove, may be melted and the glassine paper dipped thereinto or, alternatively, the glassine paper in the form of a continuous sheet may be passed through a molten bath of the addition compound, after which the resulting sheet may be calendered or supercalendered in accordance with usual practices in the art. As I have stated above, the addition componnd of urea and sodium nitrate melts at 83 degrees C. and, in the utilization of this compound for the softening of glassine, I prefer to maintain the molten bath ofthe addition compound at a temperature of the order of 85 degrees C. or not substantially in excess thereof. In the case of the use of the addition compound in molten form as described, I may treat a finished glassine paper, that is, one that has been already supercalendered, after which the temperature of the treated paper is maintained at slightly above the melting temperature of the addition compound unti1 said addition compound is absorbed into the paper. The resulting glassine paper may, although it need not, again be passed through the supercalender rolls. I prefer, however, to treat the dried, uncalendered glassine paper with the molten addition compound and, preferably while the paper is still hot, then pass the same through the supercalender rolls. I may, although it is not necessary to do so, moisten or dampen the treated paper with water prior to running said treated paper through the supercalender according to known or usual supercalendering practices.

Instead ofapplying the addition compound in molten form to the glassine paper as described above, I may dissolve said addition compound in water and apply the resulting aqueous solution to the glassine paper. In such cases, I dissolve the addition compound in such an amount of water, in the ordinary case making a solution of from about to about 30% concentration, so that the paper will take up from about 3% to about 20% of the plasticizer by weight of the finished glassine paper. Excellent results are obtained, for example, by dissolving the addition 4 compound of 100 parts of urea and 47.2 parts of sodium nitrate in approximately 500 parts of water. The application of such a solution permits a retention of about 15% of plasticizer in the finished glassine paper.

Where the addition compounds are utilized in the form of aqueous solutions, said solutions may be applied to the glassine paper, prior to the drying thereof, in any of the conventional methods by which plasticizer compositions have heretofore been applied to glassine paper as, for example, by sprays, rolls. baths, or the like. After the paper, thus treated with the plasticizer composition, is dried, I then prefer to moisten the same with water, preferably with from about 10% to about 20% of water and, as the moistened paper is wound up on the rolls, it is allowed to pass over a hot roll or prayed by live steam so as to raise its temperature to about 85 degrees C. or not substantially above 85 degrees C. or, in general to a temperature slightly above the melting point of the addition compound per se. The paper is then allowed to lie in this heated condition for several hours, after which it is supercalendered in accordance with usual practice. By so proceeding, exceptionally satisfactory resuits are obtained, namely, of the character produced by applying the addition compound in the molten state to the glassine paper as previously described. Although certain advantages of the present invention are obtained even though the heating step referred to hereinabove is not utilized, I have found that much more satisfactory results are obtained if the olution of the addition compound is heated while in contact with the paper. s previously described. The heating step apparently results in more eifectively uniting the urea with the cellulose fibers of the glassine paper to bring about thedesired softening action. Instead of applying all of the plasticizer, in the form of an aqueous solution as desribed, at once, it may be applied at different stages in the manufacture of the glassine ppaer, as, for example,

a part may be applied before the paper web is fully dried and'then, after the drying operation is completed, the balance of the plasticizer is applied at the dampener prior to the final supercalendering step.

I have also found that it is not necessary, in the preparation of the addition compounds, such as, for example, that of urea and sodium nitrate, to melt the ingredients together. Instead, in the illustrative example given, theurea and sodium nitrate, in the specified molal ratios, may be dissolved in water and applied to the paper as described above.

It will be understood that the addition compounds of urea with the nitrates, which are utilized in accordance with the present invention, may be prepared by either of the procedures described above, namely, by the dissolution of the nitrate in the molten urea or by the dissolving of v the urea and the nitrate in water, in each case in the requisite molal ratios. In all cases for obtaining the best results, the glassine paper and the addition, compound should, at some stage while in contact with each other, be heated at a temperature not below the melting point of the addition compound. This is accomplished, of course, by the application of the addition compound in molten form to the paper and, in this connection, it will be appreciated that diiferent addition compounds will have different melting points. This may also be accomplished, where the addition compound is applied to the glassine ratios, as described hereinabove, is to be sharply distinguished from prior practices where urea and agents of a hygroscopic character have been utilized. Plasticized glassine papers made pursuant to my present invention remain soft and pliable over prolonged periods of time and under conditions of low .relative humidity not through the mechanism of the retention of absorbed moisture but, rather, apparently as a result of some change in the nature or character of the cellulose fibers in the glassine paper. Although the mechanism of the action which takes place has not been fully ascertained, present indications are that some reaction occurs between the cellulose fibers of the glassine paper and the urea of the addition compound, particularly where the paper is maintained in contact with the addition compound at a temperature above the melting point of said addition compound, with the result that the character of the cellulose fibers is modified in a. manner such that the glassine paper acquires the property of remaining soft and pliable over prolonged periods of time independently of conditions of relative humidity.

The plasticized sheets of glassine may be coated with wax or other thermoplastic coating materials with no ill or adverse effect upon the color or transparency of the paper and with no lessening of the softening efiect of the addition compounds on said sheets.

It will be understood that while I have referred herein in particular to glassine, my invention is applicable to other papers having similar or analogous characteristics, such as imitation parchment and the like, at least most of which are prepared from highly beaten or gelatinized or hydrated cellulosic fibers. The term glassine, as used in the claims, is, therefore, to be understood to have this generic connotation,

What I claim as new and desire to protect by Letters Patent of the United States is:

1. Glassine paper plasticized with a plasticizer an essential ingredient of which is an addition compound of urea with a water-soluble nitrate, the molal ratio of the urea to the said watersoluble nitrate in said addition compound being not substantially less than 3 to 1,

2. Glassine paper plasticized with a plasticizer an essential ingredient 01' which is an addition compound of urea with an alkali metal nitrate, the molal ratio 01 the urea to the alkali metal nitrate in said addition compound being approximately 3 to 1.

3. Glassine paper plasticized with a plasticizer an essential ingredient of which i an addition compound of urea with an alkali metal nitrate, the molal ratio of the urea to the alkali metal nitrate in said addition compound being approximately 3 to 1, said glassine paper carrying from about 3% to about 20% of said plasticizer, based on the weight of the finished glassine paper.

4. Glassine paper plasticized with a plasticizer an essential ingredient of which i an addition compound of urea with sodium nitrate, the molal ratio of the urea to the sodium nitrate in said addition compound being approximately 3 to 1.

5. A method of plasticizing glassine paper which includes the step of contacting said glassine paper with a plasticizer comprising an addition compoundof urea with a water-soluble nitrate, the molal ratio or the urea to the watersoluble nitrate in said addition compound being not substantially less than 3 to 1.

6. A method of plasticizing glassine paper which includes the step of contacting said glassine paper with a plasticizer comprising an addition compound of urea with an alkali metal nitrate, the molal ratio of the urea to the alkali metal nitrate in said addition compound being approximately 3 to 1, and maintaining said paper, while in contact with said addition compound, at a temperature slightly above the melting point of said addition compound for a, substantial period of time.

7. A method of plasticizing glassine paper which comprises contacting said glassine paper with a, plasticizer comprising an addition compound of urea with sodium nitrate; the molal ratio of the urea to the sodium nitrate in said addition compound being approximately 3 to 1,

r and maintaining said paper, while in contact with said addition compound, at a temperature slightly above 83 degrees C. for a substantial period of time.

8. In a method of plasticizing glassine paper, the step which comprises applying thereto an aqueous solution containing, as an essential ingredient, an addition product of urea with a water-soluble nitrate, the molal ratio of the urea to said water-soluble nitrate being not substantally less than 3 to 1.

9. In a method of plasticizing glassine paper, the step which comprises applying thereto an aqueous solution containing, as an essential ingredient, an addition product of urea with a water-soluble nitrate, the molal ratio of the urea to said water-soluble nitrate being not substantially less than 3 to 1, until from about 3% to about 20% of said plasticizer, based on the weight of the finished paper, is taken up by said paper, and subjecting the resulting paper to a temperature slightly above the melting point of said addition compound for a substantial period of time.

10. In a method of plasticizing glassine paper, the step which comprises applying thereto an aqueous solution containing, as an essential ingredient, an addition product of urea with an alkali metal nitrate, the molal ratio of the urea to said alkali metal nitrate being approximately 5' to 1, and heating said resulting paper at a temperature slightly above the melting point of said addition compound for a substantial periodof time.

11. In a method of plasticizing glassine paper, the step which comprises applying thereto an aqueous solution containing. as an essential ingredient, an addition product of urea with sodium nitrate, the molal ratio of the urea to the sodium nitrate being approximately 3 to 1, until from about 3% to about 20% of said plasticizer, based upon the weight of the finished paper, is taken up by said paper, and subjecting the resulting paper to an elevated temperature but not substantially in excess of degrees C.

12. In a method of plasticizing glassine paper, the step which comprises applying thereto a molten material an essential ingredient of which is an addition compound of urea with a watersoluble nitrate, the molal ratio of the urea to said water-soluble nitrate being not substantially less than 3 to 1.

13. In a method 01. plasticizing glassine paper,

7 the step which comprises applying thereto temperature not substantially in excess of 90 molten material an essential ingredient 0! which degrees C.. a molten material an essential inis an addition compound of urea with an alkali gredient of which is an addition compound of metal nitrate, the molal ratio of .the' urea to said area with sodium nitrate, the molal ratio of the alkali metal nitrate being approximately 3 to 1. 5 urea to the sodium nitrate being approximately 14. In a method of plasticizing glassine paper. 3 to 1. the step which comprise applying thereto, at a HERBERT B. WENBERG. 

